1. Field of the Invention
The present invention relates to a small-sized antenna module, and particularly to an antenna module incorporated into portable radio equipment.
In radio equipment, an antenna is indispensable for the transmission and reception of radio wave. Particularly in small portable radio equipment, it is necessary to have a small-sized antenna with a high sensitivity built in the body of the equipment.
2. Related Prior Art
As a small antenna, an inverted F-type antenna and an S-type antenna have been peviously known. The summarized structure and the function of such antennas are disclosed, e. g. in FIGS. 1 and 2 of the prior U.S. patent application Ser. No. 858,209, filed in the Patent Office on 26, Mar., 1992 by some of the present applicants.
Those conventional inverted F-type antenna and S-type antenna are, as described in the above U.S. patent application, produced by working a conductive plate with difficulties in obtaining a desirable characteristic in respect of miniaturization and dimensional accuracy.
The applicants have thus proposed a structure and a process of producing an antenna module formed by a dielectric substrate such as a resin block in the above U.S. patent application.
The structure of the previously proposed antenna module is illustrated in FIG. 18.
In FIG. 18, (A) is a perspective view of such an antenna module wherein a dielectric substrate 1 is formed of e.g. as a thermosetting epoxy resin, thermoplastic polyether sulfone, or polyester, etc. used for an semiconductor IC mold.
A grounding conductor 2 is formed on a bottom face of the dielectric substrate 1.
An antenna element conductor 3 has a closed loop portion 31 folded at the end thereof. This type of antenna with the closed loop portion 31 is called a P-type antenna and has a leading portion 35 which also has a grounding line 36 and a feeder 37 electrically connected to the antenna element conductor 3, respectively. The space between the conducting line 36 and the feeder 37 determines the input impedance of the antenna.
In FIG. 18, (B) is a cross sectional view cut along the line A-A' in (A) of FIG. 18. As illustrated, the dielectric substrate 1 is box-shaped with a vacant space 6. A resonance frequency f.sub.c is varied depending on a size of the vacant space 6 as will be described hereinafter.
In FIG. 18, (C) shows a part of a cross sectional view of the grounding conductor 2, the antenna element conductor 3, and the leading portion 35, in which a substrate 20 of a flexible printed-circuit (FPC) is formed of a plastic film, e.g. polyimide film. On the plastic film 20, a metallic leaf such as a cupper leaf 21 is applied.
For one example, the plastic film 20 which is a substrate of FPC may be a polyimide film with a thickness of 50 .mu.m and the cupper leaf 21 with a thickness of 35 .mu.m. The size of the antenna module in FIG. 18 has a height of about 11 mm, a width of 15 mm and a length of 30-35 mm, and has a resonant frequency f.sub.c in the frequency band 800-900 MHz.
In the electric equivalent circuit of the previously proposed antenna module, the antenna element conductor portion forms an inductance L, and the space between the antenna element conductor 3 and the grounding conductor 2 corresponds to a capacitance C connected in parallel with the inductance L.
Therefore, the inductance L of the resonant frequency of the antenna module is determined by the longitudinal and lateral dimensions of the antenna element conductor 3 while the capacitance C is determined by the space between the antenna element conductor 3 and the grounding conductor 2, so that changing the area of the antenna element conductor 3 or the space between the antenna element conductor 3 and the grounding conductor 2 would change the resonant frequency f.sub.c.
In the arrangement of the previously proposed antenna module thus described, some adjustment in respect to an accuracy of .mu.m unit is required after the formation of the antenna module for a sufficient characteristic. The accuracy in forming the dielectric substrate 1 is important for its antenna module in the entirety, resulting in the increase of producing cost.
On the other hand, current flowing in a high frequency antenna as shown in FIG. 18 which is operated in the inverted F type antenna mode is concentrated in the peripheral portion of the antenna element conductor 3. This is considered in principle as phenomena due to a skin effect of the high frequency current.
In the previously proposed antenna module of FIG. 18, there is a dielectric resin provided at the side walls of the antenna element so that particularly the equivalent dielectric constant, as one of the antenna characteristics becomes high. As a result, the realization of a broad frequency band for the antenna element is prevented.
Furthermore, the above described antenna module forms a single unit used only for the transmission or reception of radio wave. It is difficult to obtain a desirable frequency band for both, radio transmisson and reception.